1. Field of the Invention
The present invention relates to a semiconductor photonic device including a semiconductor laser device for use as a light source for optical information processing, a signal source for optical communication and a pumping source for a fiber amplifier, a light emitting diode device, a semiconductor amplifier device, a semiconductor modulator and the like, and to a coating film for use in the semiconductor photonic device.
2. Description of the Background Art
In general, a coating film is formed on an end surface of a semiconductor photonic element in a semiconductor photonic device such as a semiconductor laser device and a light emitting diode device for purposes of protection and reflectivity adjustment. Conventionally, various studies have been done on the power reflectivity of such a coating film.
A technique for forming an anti-reflection coating film on an end surface of a semiconductor laser element is disclosed, for example, in: K. Shigihara et al., “Antireflection coating for laser diodes,” ELECTRONICS LETTERS, 31 Aug. 1995, Vol. 31, No. 18, pp. 1574-1576; and Japanese Patent Application Laid-Open No. 5-243689 (1993). According to this technique, if a single-layer coating film having a refractive index n1 and a thickness d1 is formed on an end surface of a semiconductor laser having an effective refractive index nc, the coating film becomes an anti-reflection film when the following conditions are satisfied: n1=nc1/2 and d1=λ0/(4·n1), where λ0 is the value of the lasing wavelength of the semiconductor laser. At this time, the magnitude of the reflection amplitude vector of the coating film is zero. The “reflection amplitude vector” used herein refers to the amplitude vector of a reflected wave obtained when the amplitude vector of an incident wave (referred to hereinafter as an “incident amplitude vector”) is placed on the positive real axis in a complex plane and is defined to have a magnitude of “1.” Thus, the reflection amplitude vector is a vector indicative of the position of an amplitude reflectivity in the complex plane.
A technique for improving the design flexibility of an anti-reflection coating film is disclosed in Japanese Patent Application Laid-Open No. 2004-88049.
A coating film having a power reflectivity greater than zero is disclosed in Japanese Patent Application Laid-Open No. 2004-289108. According to Japanese Patent Application Laid-Open No. 2004-289108, if the coating film is a single-layer film, the power reflectivity of the coating film takes on a relative minimum value not equal to zero when the following conditions are satisfied: n1≠nc1/2 and d1=λ0/(4·n1)×m (where m is an odd number). Thus, when n1≠nc1/2 and the thickness of the coating film is an odd multiple of λ0/(4·n1), the reflection amplitude vector of the coating film is present on the negative real axis in a complex plane. In this case, there is a phase difference of 180 degrees between the incident amplitude vector and the reflection amplitude vector. Then, the imaginary components of the incident and reflection amplitude vectors are equal to zero, and a difference between the real components of the vectors is equal to a power transmissivity. The value of the power reflectivity is obtained by subtracting the power transmissivity from one.
For designing the coating film having a power reflectivity greater than zero, the conventional techniques establish the condition that the thickness of the coating film is an odd multiple of λ0/(4·n1) as described above to result in the reflection amplitude vector positioned on the real axis in the complex plane. It is hence necessary to determine the thickness of the coating film so that the amplitude reflectivity is a real number when designing the thickness of the coating film having the power reflectivity greater than zero. This results in a low degree of design flexibility of the thickness of the coating film, and creates a likelihood that the coating film having a desired characteristic is not designed.